Many antennas are designed to interact primarily with electromagnetic energy propagating in a primary direction of propagation. The radiation pattern for such an antenna will generally exhibit a central lobe centered on the primary direction of propagation and some sidelobes at relatively large angles as measured from the primary direction of propagation. Depending upon the particular application for which the antenna is to be used, the presence of these sidelobes will, to a greater or lesser extent, degrade the performance of the antenna. Power levels in sidelobes may generally be reduced by redesign of the antenna. In many cases this will require a larger antenna which may be impossible to use for the particular application. In any case, the redesigned and more complex antenna will be more expensive.
Multi-layered dielectric spatial filters are known in the art for reducing sidelobes of existing antennas. In general, dielectric spatial filters comprise multiple layers of dielectric material having alternate high and low dielectric constants. The Brewster effect, which results in a decreased reflection coefficient at and near a certain angle called the Brewster angle, causes a serious degradation in the performance of multilayer dielectric spatial filters at and near the Brewster angle. The resulting passband in the filter response allows large angle sidelobes to pass which should be rejected. Other large angle passbands in the filter response curve are caused by cavity resonances, which occur at angles at which the distance between adjacent layers of the filter is an integral multiple of one-half wavelength. These two effects combine to seriously impair the ability of multilayer dielectric spatial filters to reject large angle sidelobes.